In testing breath alcohol to determine blood alcohol (ethanol) concentrations, pure deep lung breath or alveolar breath is the part of a breath sample which best reflects the actual blood alcohol concentration of the subject at the time of testing. In current commercially available breath alcohol testing devices various methods are used to insure that the portion of the breath sample that is analyzed is deep lung breath which is uncontaminated by extraneous residual alcohol in the subject's mouth or upper respiratory tract.
When a subject exhales, his or her breath initially comes primarily from the mouth and upper respiratory tract. As exhalation continues, the breath predominantly comes from the deep lungs or alveoli.
When a subject who has not recently ingested alcohol exhales, the profile of alcohol concentration in his or her breath is characterized by a rapid increase in the concentration of alcohol present, leveling off to a plateau as the breath sample becomes pure deep lung (alveolar) breath. An instrument which continuously measures alcohol concentration during exhalation, such as an infrared detector, produces an output such as shown in FIG. 1 for such a subject. This phenomenon is well known, and by identifying the level of the alcohol concentration plateau a reliable determination of blood alcohol concentration can be made.
The accurate determination of alveolar alcohol can be complicated, however, by the presence of alcohol in the patient's upper respiratory tract ("mouth alcohol"). When a subject who has recently ingested alcohol breathes into an instrument which continuously measures alcohol concentration, the output profile is characterized by a more pronounced and rapid increase in the sample's alcohol concentration followed by a decrease in the indicated alcohol concentration as shown in FIG. 2. The conventional way to determine the presence of mouth alcohol is to measure the output of the detector during the entire exhalation of a subject's breath, to look for this characteristic peak in the indicated concentration of alcohol, and to reject the sample if this peak is present.
It is known to those skilled in the art that this approach is not entirely reliable. As the amount of mouth alcohol present declines, the characteristic peak in alcohol concentration becomes less and less apparent until, at lower levels, mouth alcohol is completely indistinguishable from deep lung alcohol. Another difficulty arises when a subject with a large amount of alcohol in his or her upper respiratory tract provides a slow and steady breath sample. It is possible that the residual alcohol in such a subject never gets completely flushed from the upper respiratory tract and no peak ever occurs. The characteristic peak in alcohol concentration from mouth alcohol in both of these cases is further obscured by the presence of actual blood alcohol in the deep lung air.